The present invention relates to sheet post processing apparatuses that carry out post processing on the sheets discharged after image forming by image forming apparatuses such as electro-photographic copiers, printers, facsimile units, and multi-functional units having all these functions, and also relates to image forming systems.
Multi-functional image forming systems are coming into wide use in which sheet post processing apparatuses that carry out various types of post-processing on sheets are connected to image forming apparatuses of the electro-photographic type thereby making it possible to carry out various types of post processing processes such as folding, stitching, etc., on sheets on which images have been formed.
In the case of conventional folding processes, in order to produce a booklet such as that shown in FIG. 12, the sheets were center-folded at one folding position, and thereafter, saddle stitching was carried out.
In the above method, since the saddle stitching process is carried out by stapling after a bundle is prepared by stacking individually center-folded sheets, in cases when the sheets are of grain direction, or when the sheets are thick, or when the number of sheets is large, etc., if the size of the sheets is small, there is the problem that, as is shown in FIG. 12, because of the strength of the paper, the stapled part of the booklet gets bulged or the booklet gets opened in two parts in the direction of the arrow F.
FIG. 12 is a diagram showing the state in which the stapled part of a booklet has bulged and the booklet has got opened in two parts.
In order to solve this type of problems, a technology has been proposed, in United States Patent Application Publication No. US2003/0031532A1, in which a square shaped back of booklet is made by pressing the back part of the booklet using a roller after the stack of sheets is center-folded.
Patent Document 1: United States Patent Application Publication No. US2003/0031532A1
However, in the above proposed method a plurality of sheets are made into a bundle, and after that bundle of sheets has been center-folded, the back part being the central part of a booklet is formed into a square shape, and bulging of the booklet cannot be avoided.